Secondary batteries and electrochemical capacitors are known as power storage devices. For example, secondary batteries include lithium ion secondary batteries, nickel hydrogen batteries, or lead storage batteries. Electrochemical capacitors include electric double layer capacitors. Due to size reduction of cellular phones, or restriction of installation spaces, for example, power storage devices are desired to be made much smaller. Accordingly, lithium ion batteries with high energy density are gathering attention. As packaging materials for lithium ion batteries, metal cans have conventionally been used. However, a recent trend is to use multi-layer films as such packaging materials, which are lightweight, have high heat dissipation, and can be produced at low cost.
The electrolytic solution of such a lithium ion secondary battery is configured by an aprotic solvent, such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, and an electrolyte. As the electrolyte, a lithium salt, such as LiPF6 or LiBF4, is used. However, these lithium salts generate hydrofluoric acid by a hydrolysis reaction. The hydrofluoric acid may corrode the metal surface of the battery member, or degrade the lamination strength between the layers of the multi-layer film serving as a packaging material.
To address such corrosion and degradation, the aforementioned packaging material is provided with an aluminum foil or the like as a barrier layer on the inside of the multi-layer film to prevent penetration of moisture from the surface of the multi-layer film. A multi-layer film known as the aforementioned packaging material includes, for example, a heat resistant base material layer, a first adhesive layer, a barrier layer, an anti-corrosion treatment layer preventing corrosion due to hydrofluoric acid, a second adhesive layer, and a sealant layer, which are laminated in this order. A lithium ion secondary battery using a packaging material provided with an aluminum foil as a barrier layer, as mentioned above, is also referred to as an aluminum laminate type lithium ion secondary battery.
For example, the aluminum laminate type lithium ion secondary battery can be obtained by a method of cold forming a recess on part of a packaging material, accommodating battery elements, such as a positive electrode, a separator, a negative electrode, and an electrolytic solution, in the recess, folding back the rest of the packaging material, and bonding the edge portions by heat sealing. Such a lithium ion secondary battery is also referred to as an embossed lithium ion secondary battery. To enhance energy density, the embossed lithium ion secondary batteries manufactured in recent years are provided with recesses on both sides of the packaging material to be bonded to accommodate more battery elements.
The energy density of such a lithium ion secondary battery increases more, as a recess formed by cold forming becomes deeper. However, as the recess formed becomes deeper, pinholes or breakage are more likely to occur in the packaging material during forming. As a measure against this, a stretched film is used for the base material layer of the packaging material to protect the metal foil (e.g. see PTLs 1 and 2).